1. Technical Field
The present invention relates to precision measurement tools and, more particularly, to a shared control bus between a host and a precision measurement assembly.
2. Discussion of Related Art
Precision measurement tools include coordinate measuring machines (CMM), precision measuring instruments and the like. CMMs range from bench top and articulated arm manual CMMs to high-speed DCC scanning machines, gantries, shop floor measuring robots and horizontal arm CMMs complete with metrology software, probes and accessories with support networks.
For instance, FIG. 1 shows a precision measurement assembly 10 in the form of a vertical CMM, according to the prior art. The assembly includes a stationary platform 12 and primary or first a movable part 14 mounted thereon. In the case illustrated of a vertical CMM, the first movable part 14 takes the form of a bridge which bridges the platform 12. Further configurations such as horizontal CMMs, the first movable part will typically take another form such as simply a vertical column. In any event, for the illustrated case of a vertical CMM, an actuator is mounted either on the platform 12 or the bridge 14 for moving the bridge 14 with respect to the platform along an axis such as an axis 15 parallel to one of the edges of the platform 12. The bridge 14 may also typically have one or more other movable parts such as a second moveable part or carriage 16 which is actuated by a corresponding actuator mounted on the carriage 16 itself or on the bridge 14. The carriage 16 may be moved along an axis such as an axis 17 perpendicular to the axis 15. One of these carriages 16 will have a probe 18 mounted thereon for being moved along a surface of an object situated on the platform. A probe axis 19 may be perpendicular to both the axes 15, 17 and the carriage 16 may be moved by an associated actuator along the axis 19. The various actuators execute controlled movements of the at least one carriage 16 and the bridge 14 so as to cause the probe to move relative to the surface of the object to be measured along a preplanned path. There are a number of position sensors associated with the carriage 16, bridge 14 and platform 12 for sensing the position of the bridge relative to the platform and the carriage 16 relative to the bridge and for providing signals having magnitudes indicative thereof. Similarly, the probe 18 is associated with a position sensor which provides a signal having a magnitude indicative of the position of the tip of the probe 18 relative to the carriage 16, e.g., along an axis 19 of the probe or carrier.
Various coordinate transformations may be carried out to translate the position of the probe into a surface map of the object to be measured. The surface of the object is of course known to a large degree in advance on account of a CAD program or the like stored in a host CPU 22 having a display 24 and user input device 26 such as a keyboard and/or mouse. The user will utilize the preexisting CAD representation of the object to be measured using a software interface program to create the preplanned path along which the probe 20 is to be moved. The objective is to measure the surface of the object with great precision. Such a host CPU may be connected to the precision measurement assembly directly or a via a controller 28 containing relays, power supplies, and other hardware which would not normally be present in a host CPU and which would be better situated separate from the precision measurement assembly 10. Such a controller might be connected to the host CPU 22 by means of an RS 232 interface 32, one or more twisted pairs comprising an ethernet connection 34 and/or any generalized connection symbolized by the reference numeral 30. One of the disadvantages of having the controller separate from the precision measurement assembly 10 is that numerous wires 36 have to be utilized to interconnect the controller to the precision measurement assembly 10, particularly the bridge or other equivalent first moveable part. In a typical example, there might be numerous temperature sensors mounted on the precision measurement assembly which have to be connected to the controller 28 by as many as 20 wires as shown. Similarly, other types of devices such as multiple motors, servos, encoders, probes and switches may be associated with the precision measurement assembly and need to be connected electrically to the controller 28 by means of wires. Also, a power supply within the controller 28 has to provide power on additional wires which may be of fairly heavy gauge to the precision measurement assembly 10. All these wires add up to a significant number. In the example shown, a total number of wires of 113 is required. These wires are heavy and have to be enclosed within a flexible conduit called an energy track which is designed to smoothly uncoil and coil the wires as the bridge 14 moves with respect to the platform 12. This energy track is normally situated on one of the sides of the platform 12 at one end of the first moveable part 14, in this case the bridge 14.
The large number of wires creates a significant cable drag problem in view of the fact that even the very slightest twist in the bridge caused by such drag will cause a deformation from the mathematical model of the ideal system to such an extent that a significant imprecision in the measurement is introduced. It is also the case that the controller itself 28 is typically designed in the present state of the art according to a fairly obsolete bus architecture (ISA) and it will be desirable to modernize the controller itself. Another problem is limited servo performance which it will be desirable to improve.